Caveolin

Caveolae are invaginations of the plasma membrane. They contain the protein caveolin and are rich in certain phospholipids. Similar to coated pits, they bud off internally forming endocytic vesicles. Caveolae play an important role in the internalization of certain cell surface receptors. 

A constitutive enzyme binding to caveolin-1 in the plasma membrane, mainly in endothelial cells. It is activated in the presence of Ca2+ and calmodulin. 

Similar to nNOS, Ca2+-activated calmodulin is important for the regulation of eNOS activity. However, several other proteins interact with eNOS and regulate its activity. Heat shock protein 90 (hsp90) is found associated with eNOS and probably acts as an allosteric modulator that activates the enzyme. Caveolin-1 binds eNOS and directs it to caveolae. Caveolin-1 is viewed as an inhibitor of eNOS activity, which is being replaced by CaM upon activation of endothelial cells [2]. 

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

Uittenbogaard, A, Everson, WV, Matveev, SV, and Smart, EJ, 2002. Cholesteryl ester is transported from caveolae to internal membranes as part of a caveolin-annexin II lipid-protein complex. J Biol Chem 277,4925—4-931. 

Since endocytosis ofLDH was confirmed by TEM images (Figure 13.9), forthe next step, its specific endocytic pathway for membrane entry was determined by immunofluorescence and confocal microscopy. Cells were incubated with LDH-FITC, fixed with 3.7% freshly made formaldehyde, and then stained with either anti-clathrin antibody or anti-caveolin-1 antibody both conjugated to the red fluorescent dye Texas Red (TR). The confocal microscopic images showed that green fluorescent... 

LDH-FITC is well overlapped with red fluorescent clathrin-TR, but not with caveolin-1-TR (Figure 13.10). This is dear evidence that clathrin-mediated endocytosis is the prindpal mechanism for the cellular internalization of LDH particles. Caveolae-mediated endocytosis, if any, seems not to be responsible for LDH uptake. 

Other systems like electroporation have no lipids that might help in membrane sealing or fusion for direct transfer of the nucleic acid across membranes they have to generate transient pores, a process where efficiency is usually directly correlated with membrane destruction and cytotoxicity. Alternatively, like for the majority of polymer-based polyplexes, cellular uptake proceeds by clathrin- or caveolin-dependent and related endocytic pathways [152-156]. The polyplexes end up inside endosomes, and the membrane disruption happens in intracellular vesicles. It is noteworthy that several observed uptake processes may not be functional in delivery of bioactive material. Subsequent intracellular obstacles may render a specific pathway into a dead end [151, 154, 156]. With time, endosomal vesicles become slightly acidic (pH 5-6) and finally fuse with and mature into lysosomes. Therefore, polyplexes have to escape into the cytosol to avoid the nucleic acid-degrading lysosomal environment, and to deliver the therapeutic nucleic acid to the active site. Either the carrier polymer or a conjugated endosomolytic domain has to mediate this process [157], which involves local lipid membrane perturbation. Such a lipid membrane interaction could be a toxic event if occurring at the cell surface or mitochondrial membrane. Thus, polymers that show an endosome-specific membrane activity are favorable. 

Schlachetzki, F. and Pardridge, W. M. P-glycoprotein and caveolin-lain endothelium and astrocytes of primate brain. Neuroreport 14 2041-2046, 2003. 

Other pinocytotic pathways also exist that do not depend on either caveolae or clathrin, although these are not as well defined [55]. Specific receptors continue to be internalized in the absence of clathrin or caveolin and these pathways can be monitored by following glycosyl phos-phatidylinositol (GPI (-anchored proteins. Nonclathrin, noncaveolin pathways may also be responsible for the reuptake of membrane in neuroendocrine cells after stimulated secretion. Some, but not all, of these pathways appear to require dynamin. 

Taggart MJ 2001 Smooth muscle excitation-contraction coupling a role for caveolae and caveolins News Physiol Sci 16 61-65... 

Parton, R. G. (1996). Caveolae and caveolins, Curr. Opirt. Cell Biol., 8, 542-548. 

Despite the weakness and short-range nature of protein-lipid and lipid-lipid interactions, cells have nevertheless evolved means of laterally assembling into membrane-mi-crodomains. Sphingolipid-cholesterol rafts serve to recmit a specific set of membrane proteins and exclude others [24]. Caveolae are deeply invaginated raft domains that are stabilized by caveolin protein oligomers (binding cholesterol) [25]. 

Ehrhardt C, Kneuer C, Laue M, Schaefer UF, Kim KJ, Lehr CM (2003) 16HBE14o- human bronchial epithelial cell layers express P-glycoprotein, lung resistance-related protein, and caveolin-1. Pharm Res 20(4) 545-551. 

The rat cell line R3/1 was established from cells obtained from broncho-alveolar tissues of foetal Wistar rats at 20 days of gestation. This cell line displays a phenotype with several characteristic features of ATI cells. R3/1 cells were analysed to show a positive expression for mRNA and protein content of markers related to the ATI cell type (e.g., Tla, ICAM-1, connexin-43 and caveolin-1 and -2) [79], Whether or not this cell line can form functional tight junctions is currently under investigation in our laboratories. 

Newman GR, Campbell L, von Ruhland C, Jasani B, Gumbleton M (1999) Caveolin and its cellular and subcellular immunolocalisation in lung alveolar epithelium implications for alveolar epithelial type I cell function. Cell Tissue Res 295(1) 111-120... 

Kasper M, Reimann T, Hempel U, Wenzel KW, Bierhaus A, Schuh D, Dimmer V, Haroske G, Muller M (1998) Loss of caveolin expression in type I pneumocytes as an indicator of subcellular alterations during lung fibrogenesis. Histochem Cell Biol 109(1) 41—48... 

Razani B, Engelman JA, Wang XB, Schubert W, Zhang XL, Marks CB, Macaluso F, Russell RG, Li M, Pestell RG, Di Vizio D, Hou H Jr, Kneitz B, Lagaud G, Christ GJ, Edelmann W, Lisanti MP (2001) Caveolin-1 null mice are viable but show evidence of hyperproliferative and vascular abnormalities. J Biol Chem 276(41 ) 38121-38138... 

Fuchs S, Hollins AJ, Laue M, Schaefer UF, Roemer K, Gumbleton M, Lehr CM (2003) Differentiation of human alveolar epithelial cells in primary culture— Morphological characterisation and expression of caveolin-1 and surfactant protein-C. Cell Tissue Res 311 31-45... 

Koslowski R, Barth K, Augstein A, Tschernig T, Bargsten G, Aufderheide M, Kasper M (2004) A new rat type I-like alveolar epithelial cell line R3/1 bleomycin effects on caveolin expression. Histochem Cell Biol 121 (6) 509—519... 

M. G. Qaddoumi, H. J. Gukasyan, J. Davda, V. Labhasetwar, K. J. Kim, and V. H. Lee. Clathrin and caveolin-1 expression in primary pigmented rabbit conjunctival epithelial cells role in PLGA nanoparticle endocytosis. Mol Vis 9 559-568 (2003)... 

The distinguishing structural and functional protein for caveolae is caveolin. Caveolin proteins display properties that are likely involved in the distinguishing morphology of caveolae. Caveolins have a high affinity for both cholesterol and sphingolipids coupled with 3 carboxy-terminal palmitoylated cysteine residues. Three isoforms of caveolin exist and show distinct tissue distribution. Likely because it was discovered first and is perhaps most abundant, caveolin-1 has garnered the lion s share of research attention. 

Caveloae are particularly abundant, accounting for 30-70% of the plasma membrane in differentiated epithelial (e.g., pneumocytes) and endothelial cells, fibroblasts, smooth muscle cells, and adipocytes. Indeed there is a general trend for caveolin induction in differentiated cell types. Adipose tissue is replete with caveolae, and caveolin mRNA and protein are strongly induced during differentiation of 3T3-L1 preadipocytes (fibroblasts) to adipocytes [10], While... 

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